Xylene isomers consist of para-xylene, meta-xylene, ortho-xylene and ethylbenzene. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers, such as polyester. Meta-xylene is used for the manufacture of insecticides, isophthalic acid or alkyd resins. Ortho-xylene can be used as material for plasticizers.
Xylenes are found in substantial quantities in coke oven light oils and certain virgin and reformed petroleum naphthas. In the past, it has been the practice in the separation of xylene isomers either to use chemical methods or to distill meta- and para-xylene, ethylbenzene and some paraffins from a fraction containing the xylene isomers, ethylbenzene, and paraffins. Typically, para-xylene and ortho-xylene are produced by recovery of these isomers from a mixed C.sub.8 aromatic stream by means of fractionation, adsorption or crystallization.
The composition of the C.sub.8 aromatic stream will vary depending on its source, but the total ortho- and para-xylene isomer content will be less than about 45 percent. Thus, a recovery process alone will not fully utilize the valuable feedstock. Under these circumstances, isomerization is useful. After depleting the C.sub.8 aromatic feedstock of the valuable isomers (ortho- and para-xylene), the raffinate is generally sent to an isomerization unit. In the isomerization unit, an equilibrium xylene isomer ratio is reestablished, thus producing the desired isomers from the undesirable ones.
One method of producing a para-xylene product from a C.sub.8 aromatic mixture is to first pass the C.sub.8 mixture to a xylene column to remove heavies, for example, C.sub.9.sup.+ hydrocarbons. The overhead stream from the xylene column, containing predominantly para- and meta-xylene and ethylbenzene is passed to a crystallization unit. The crystallization unit is a separation process that takes advantage of the fact that for most xylene mixtures the melting point of para-xylene is higher than the other xylene isomers and crystallizes first. For example, para-xylene crystallizes at 13.3.degree. C., meta-xylene at -47.9.degree. C. and ortho-xylene -25.2.degree. C. In the physical system of the three xylene isomers, there are two binary eutectics of importance, the para-xylene/meta-xylene and the para-xylene/ortho-xylene. As the para-xylene is crystallized from the mixture, the remaining mixture (mother liquor) composition approaches one of these eutectic binaries depending on the starting composition of the mixture. In commercial practice, para-xylene is crystallized so that the binary is only approached but not reached to avoid contaminating the crystal body with a mixture of crystals. This represents the eutectic limit to recovering para-xylene by crystallization. The crystallization unit produces a mother liquor which is recycled to the isomerization unit where the composition of the mother liquor is restored to the approximate concentration of the initial C.sub.8 aromatic feedstock to form additional para-xylene.
The key disadvantage of crystallization is that the efficiency of the crystallization unit is restricted by the eutectic limit of the feed to the crystallization unit. A maximum extraction efficiency of about 65% is typical of commercial crystallization operations.
The eutectic limit problem of the crystallization process has been recognized in the C.sub.8 aromatic separation art. In U.S. Pat. No. 3,939,221 (issued to Pearce), it is taught that the eutectic point of the crystallization unit could be overcome by passing the mother liquor generated by the crystallization unit through a selective adsorption unit prior to contacting the mother liquor with an isomerization catalyst to convert additional xylene isomers to para-xylene. The adsorption unit contains a crystalline aluminosilicate active for the selective adsorption of para-xylene from the mother liquor. Nowhere in the Pearce patent is there disclosed or suggested introducing a C.sub.8 aromatic stream into a selective adsorption zone to provide a para-xylene-enriched stream for crystallization.
In U.S. Pat. No. 3,729,523, a process is disclosed for recovering high purity streams of all of the C.sub.8 aromatic xylene isomers including ethylbenzene. The process passes a C.sub.8 aromatic feed stream containing all of the xylene isomers through a selective adsorption unit to produce one stream containing meta- and ortho-xylene and another stream containing para-xylene and ethylbenzene. The stream containing the para-xylene and ethylbenzene is passed to a crystallization unit for removal of para-xylene and then to a first distillation zone for removal of ethylbenzene. The stream containing meta- and ortho-xylene is passed to a second distillation zone for removal of meta- and ortho-xylene. Also exiting this second distillation zone is a residual C.sub.8 aromatic mixture that is sent to a low temperature isomerization zone (with toluene dilution) to form additional para-xylene.
There is a need for a cost effective method of producing high purity para-xylene from a C.sub.8 aromatic mixture containing para-xylene, meta-xylene, ortho-xylene and ethylbenzene which overcomes the aforementioned eutectic limit for crystallization.